A system in which the same data is stored in a plurality of storage devices (which will hereinafter be referred to as a replication system) is known as a storage system. A configuration and functions of the existing replication system will hereinafter be described by use of FIGS. 1 through 4.
As schematically illustrated in FIG. 1, the existing replication system includes N number (N=4 in FIG. 1) of storage devices and N number of nodes which are connected to a host (host computer) via a network and each of which is connected to the storage device different from each other. Each node in this replication system is basically a computer which performs control (read/write access) with contents corresponding to a read/write request given from the host over the storage device connected to the self-node. Each node, however, in the case of receiving the write request of a certain item of data under a condition that a setting such as “Replication Number=3” is done, instructs a next node determined from a hash value etc. of identifying information of the data to store the data in the storage device. Then, the node receiving the write request and the two nodes receiving the instruction from another node store the same data in the three storage devices.
Further, the existing replication system also has a function of copying the data (which will hereinafter be termed a replication rebuilding function) so that the replication number of all the data becomes a setting value if a fault occurs in a certain storage device or a certain node.
To be specific, if the fault occurs in a node B or a storage device B of the replication system depicted in FIG. 1, as illustrated in FIG. 2, it follows that the replication Number of data 1, data 2, data 4 decreases down to “2”.
In such a case, in the existing replication system, as schematically illustrated in FIG. 3, each node executes a process of reading a specified item of data stored in the storage device B out of the storage device of the self-node, and requesting another node to store the readout data in the storage device. Then, as depicted in FIG. 4, the replication system is returned to a status where the replication Number of all the data is “3”.